In recent years, various information transmission systems such as a broadcasting system, a cable communication system and a mobile communication system have been put to practical use or planed, and as a result thereof, various terminals have been developed.
For example, in a mobile communication system, mobile terminals such as mobile phones and PDAs (Personal Digital Assistants) are used. In this kind of terminals, not only voice communication service, but also various transmission services such as SMS (Short Message Service), electronic mail service and information distribution service using wireless internet are available. When data including a large amount of information such as motion picture data (or video data) are tried to transmit in the mobile communication system, they cannot be transmitted as they are because the transmission path band causes a bottleneck.
So, a compression/extension processing of video data is required in transmitting motion picture data wirelessly, and as one means of realizing the processing, the employment of a compression/extension scheme called MPEG-4 (Moving Picture Experts Group-4) is being discussed.
MPEG-4 represents data having correlations on the time axis such as video data with an initial frame and a difference frame. The initial frame is called I-Frame (Intra-coded Frame) and contains whole picture information elements (absolute value information) of one picture obtained by being compressed as a still picture. That is, the I-Frame composes a picture frame having an independent meaning alone. In contrast thereto, the difference frame is called P-Frame (Predictive-coded Frame) and mainly contains difference information between the picture of the current frame and the picture of the preceding frame. And, in the first timing after the start of transmission an I-Frame is transmitted, and in the subsequent frame timings P-Frames are transmitted sequentially. By using MPEG-4, motion picture data having an amount of information equivalent to several Mbps can be transmitted with being compressed to several ten Kbps.
However, in a system employing such a compression/extension scheme, if the video recording button of the apparatus is pushed in receiving data, only difference frames can be recorded, and as a result thereof, it becomes difficult to reproduce pictures effectively from the recorded video data.
So, in MPEG-4, an idea that the picture of each P-Frame is divided into a large number of blocks and that absolute value information is transmitted in a part of blocks selected optionally from each P-Frame is employed. The blocks in which the absolute value information is transmitted are called a macro-block respectively. By receiving a plurality of P-Frames and synthesizing the macro-blocks thereof, the apparatus can reproduce one picture data equivalent to picture data of the I-Frame.
Therefore, even if the picture recording button of the apparatus is pushed in receiving picture data, one picture data comprising absolute value information can be reproduced by receiving the same number of P-Frames as the macro-blocks constituting one picture.
However, in a period from the start of video recording to the end of receiving the same number of P-Frames as the macro-blocks constituting one picture, one picture data comprising absolute value information cannot be reproduced. The above-described period lasts, for example, about 10 seconds. As a result thereof, users may fail to record video data of desired scenes.